Capacitive touch sensors allow a user to make a selection by touching an area of the sensor surface. Such sensors may be transparent, where a touch screen is provided over a conventional display screen, and the user touches a displayed icon to select it. Other touch sensors do not need to be transparent, since there is no display screen behind it.
For touch sensors that do not overlie a display screen, there may be a need to uniformly illuminate the sensor or identify touch positions on the sensor by illuminated areas.
U.S. Pat. No. 8,638,314 describes an electro-luminescent (EL) layer (typically a phosphorescent powder) sandwiched between upper and lower electrodes of a capacitive touch sensor, where the top electrode is transparent. The sensor may be for a single touch button or part of a larger sensor where touching different areas of the sensor signify a different selection. An electric field across the EL layer causes it to illuminate. For the touch sensor function, an increase in the mutual capacitance between the opposing electrodes indicates that the user's fingertip is touching the spot directly above the electrode. A controller rapidly time-multiplexes a high voltage EL energization voltage with a touch detection signal to multiplex the functions of the two electrodes. Therefore, the EL appears to be constantly on to illuminate the touch sensor. The shapes of the electrodes could be patterned so that the abutting EL portion illuminates with the same shape (e.g., to identify a number 0-9). One problem with such an EL layer is that a high voltage DC to AC converter is needed to supply the high voltage (over 100V) to the EL layer. The multiplexer must be designed to handle the high voltage at the high switching frequency. Further, since the optical characteristics of the phosphorescent layer are related to the thickness of the layer, and the operation of the touch sensor is also related to the gap between the electrodes, there is some trade-off between the optimal EL design and the optimal touch sensor design. Further, the EL layer becomes saturated beyond a certain voltage, so brightness control is limited. Additionally, the design only works for touch sensors of the type that sense the change in mutual capacitance between upper and lower electrodes. Thus, the EL function could not be used with a planar type of touch sensor, where all the electrodes are formed on a single layer. Other drawbacks exist.
US Patent Application Publication 2012/0206392 describes a transparent touch pad with an array of conventional packaged LEDs on a printed circuit board attached to the back of the touch pad to backlight the touch pad. Such an arrangement is expensive and relatively thick. Also, there is poor light coupling to the touch pad. Such an arrangement is analogous to the touch pad being a touch screen laminated over a display screen.
What is needed is a different design for an illuminated capacitive touch sensor that does not suffer from the drawbacks of the prior art.